The present invention pertains to a process of recovering and purifying 3,4-epoxy-1-butene (epoxybutene) from a reaction product gas, obtained by the vapor phase catalytic partial oxidation of 1,3-butadiene with oxygen over a silver catalyst. More specifically, the present invention pertains to a process of recovering epoxybutene from an epoxybutene-laden reaction product gas by absorption into a high-boiling solvent. This invention also pertains to a method of separating epoxybutene from the solvent and other reaction by-products by a novel combination of distillation and decantation steps.
Ethylene oxide (EO) and epoxybutene both may be produced in large scale plants by similar catalytic partial oxidations of the corresponding olefin with oxygen over a silver catalyst. See for example, U.S. Pat. Nos. 2,773,844 and 3,962,136, and 4,356,312 for EO and U.S. Pat. Nos. 4,897,498, 4,950,773, and 5,081,096 for epoxybutene. Considerable effort has been devoted to the development of efficient methods of recovering these epoxides, particularly EO, from the reaction product gas and subsequent purification of the epoxide.
According to U.S. Pat. Nos. 3,745,092 and 3,964,980 and Dever et al. Ethylene Oxide, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., 1994, pp. 929-930, EO is recovered and purified according to the following procedure. A reaction product gas containing typically 0.5 to 5% EO, obtained by the vapor phase catalytic oxidation of ethylene with oxygen over a silver catalyst, is introduced to an EO absorption tower where it is contacted counter-currently with an absorbent comprised mostly of water, within which the EO is absorbed. The absorber is typically maintained at a temperature of 5 to 40xc2x0 C. and 10 to 30 bars absolute (bara).
The EO-laden absorbent is then sent to a stripping column where vaporous EO is recovered from the top of the tower at a temperature of 85 to 140xc2x0 C. by steam stripping at reduced pressure. The water remaining after the distillation of EO is recycled to the absorption tower for reuse. EO reacts readily with water under absorption and distillation conditions to form ethylene glycol, which can react further to form diethylene glycol, triethylene glycol, and higher oligomers. Although ethylene glycol is a valuable and marketable chemical, diethylene glycol and higher oligomers have much less commercial demand and are thus generally undesirable by-products. Formation of ethylene glycol oligomers can be controlled to some extent by limiting ethylene glycol concentration in the recycled water to the absorber. Typical levels are less than 10 weight per cent ethylene glycol in the recycled absorber water.
The crude EO vapor recovered in the stripper overhead comprises EO as the main component, as well as impurities such as water, argon, nitrogen, carbon dioxide, methane, ethane and ethylene, formaldehyde, and acetaldehyde. The light or low-boiling components, e.g., nitrogen, carbon dioxide, argon, methane, ethane, and ethylene are removed overhead in a second stripping column. The partially purified EO is removed from the lower section of base of the second stripping column and is transferred to the mid-section of a refining column for final purification. U.S. Pat. Nos. 5,529,667 and 3,418,338 disclose the use of extractive distillation with water as a solvent in either the second stripping column or the refining column to reduce the level of aldehyde impurities in the final purified EO product.
By employing the above-described procedure, EO purities of greater than 99.5 mole per cent are possible. Although these water-based processing steps function effectively for EO recovery and purification, they cannot be employed equally efficaciously for the recovery and purification of epoxybutene. Firstly, whereas EO is completely and infinitely miscible with water, epoxybutene is only sparingly miscible with water. At 25xc2x0 C., the solubility of epoxybutene in water is only about 5 to 6 weight percent. As a result, water is a very poor absorbent for epoxybutene. High water to epoxybutene ratios, e.g., upward of 50/1 to 150/1, are required to ensure complete absorption of epoxybutene from the reaction off gas. Such ratios are prohibitive from equipment cost and energy usage standpoints.
Secondly, EO is a relatively low-boiling component compared to water, i.e., normal boiling point of 10.4xc2x0 C. versus 100xc2x0 C., respectively, and does not form an azeotrope with water. Thus, EO can be distilled readily from water by simple fractional distillation techniques as described above for the conventional EO recovery scheme. However, epoxybutene is much more hydrophobic than EO and forms a minimum-boiling azeotrope with water. High purity epoxybutene cannot be obtained by the simple fractional distillation techniques employed for EO recovery.
Other methods proposed for recovery of EO from ethylene oxidation effluents likewise are not effective or are uneconomical for epoxybutene recovery and purification. For example, U.S. Pat. No. 3,948,621 discloses a method of separating EO and carbon dioxide simultaneously from a mixed gas obtained from catalytic oxidation of ethylene by oxygen using methanol as an absorbent. As with water, epoxybutene forms a minimum-boiling azeotrope with methanol and, thus, epoxybutene and methanol cannot be separated readily by simple fractional distillation.
U.S. Pat. Nos. 4,437,938 and 4,437,939 disclose methods using supercritical or near supercritical carbon dioxide and water at the same time as absorbents. EO is first absorbed into water as in conventional recovery methods. The EO-rich aqueous absorbent contacted with (near) supercritical carbon dioxide, and EO is extracted to the carbon dioxide solvent. The carbon dioxide is separated from EO by distillation under reduced pressure. The carbon dioxide is recompressed before recycling as the extraction solvent. This method, however, has many drawbacks. First, the required amount of (near) supercritical carbon dioxide is approximately 35 times the amount of EO to be absorbed therein, resulting in large equipment. The extraction is carried out at high pressures, e.g., 86 bara, while the distillation step is carried out at lower pressure, i.e., about 0.1 to 2 bara. The wide pressure swings results in high compression costs and thus does not provide an economical solution.
U.S. Pat. Nos. 4,221,727 and 4,233,221 discloses an EO recovery method that uses ethylene carbonate as an absorbent for EO. Ethylene carbonate has many advantages as an absorbent. The absorption affinity of ethylene carbonate for EO is higher than that of water. The vapor pressure of ethylene carbonate is quite low, i.e., normal boiling point of 239xc2x0 C., so losses into the recycle gas are minimal. Moreover, ethylene carbonate is stable and does not directly react with EO. The process disclosed in U.S. Pat. No. 4,233,221, however, has the following drawbacks for EO and epoxybutene recovery. The most preferred temperature range for operation of conventional water absorption of EO is 5 to 40xc2x0 C. The melting point of ethylene carbonate is 39xc2x0 C., so ethylene carbonate would be a solid over almost all of the preferred temperature range. In order to avoid solidification it is necessary to operate the absorber and other processing equipment substantially above, i.e., at least 10 to 20xc2x0 C., above the melting point of ethylene carbonate. This is much higher temperature than an operation using water. The absorbing power of the ethylene carbonate correspondingly decreases so that the amount of circulating absorbent must be increased, reducing the economic utility of the process.
U.S. Pat. No. 5,559,255 describes the use of propylene carbonate as an absorbent for EO. The EO-laden propylene carbonate is stripped with an inert gas to recover EO and the water by-product from the epoxidation reactor as a vapor. Purified EO is produced from the mixed water-EO vapors as in conventional methods described in U.S. Pat. Nos. 3,745,092 and 3,964,980. Unlike ethylene carbonate, propylene carbonate is a liquid at room temperature and thus offers a more robust process than ethylene carbonate absorption. However, the process described In U.S. Pat. No. 5,559,255 also has drawbacks for epoxybutene recovery and purification. Epoxybutene is a much less volatile component than EO and cannot be removed effectively from propylene carbonate by inert gas stripping as described in the ""255 patent. Moreover, this EO process does not presage or address the problems associated with epoxybutene recovery and separation from the epoxybutene-water azeotrope, butadiene, or other impurities absorbed with epoxybutene from the epoxidation reactor product gas.
U.S. Pat. No. 3,644,432 discloses the use of liquid ethane as an absorbent for EO. The reactor product gas is cooled, compressed, and then passed through a molecular sieve drier bed to remove the by-product water of reaction. The dried reactor product gas is contacted in a countercurrent absorption tower with liquid ethane at a preferred temperature range of xe2x88x9231.5 to xe2x88x9217.6xc2x0 C. at a pressure of about 1.8 MPa. EO is much more soluble in liquid ethane than in water, so the solvent to feed gas ratio of the absorber can be reduced considerably from the water absorbent case, with concomitant cost reductions. However, maintenance of such cryogenic temperatures expensive refrigeration equipment and much more than offsets any savings due to lower solvent to feed gas ratios. Thus, there are no acceptable EO absorption/separation methods that can be adapted readily and economically to epoxybutene absorption/separation.
The patent literature is not as extensive for epoxybutene production, but several patents describe the recovery and separation of epoxybutene. U.S. Pat. Nos. 5,117,012 and 5,312,931 disclose the use of liquid butadiene and butadiene/butane mixtures as an absorbent for epoxybutene. The reactor product gas is cooled, compressed, and contacted in a countercurrent absorption tower with liquid butadiene/n-butane at a preferred temperature range of 0.0 to 30xc2x0 C. at a pressure of about 5 to 15 bara. Water and water-soluble impurties are removed by decantation of the epoxybutene-rich absorbent stream. Any remaining water, butadiene/n-butane absorbent, and low-boiling impurities are removed by distillation to give a purified EpB product. However, n-butane and 1,3-butadiene have relatively high volatilities, with normal boiling points of xe2x88x920.5xc2x0 C. and 4.5xc2x0 C., respectively. In order to ensure that the solvent n-butane/butadiene largely remains a liquid within the absorption zone at operating temperatures that can be achieved with an inexpensive cooling medium such as water, i.e., above at least about 30xc2x0 C., the absorption zone must be operated at a pressure of at least about 4.2 bara. Operation at lower pressures, and concomitantly lower temperatures is quite costly if the required low temperature cooling is supplied by ordinary means to those skilled in the art such as chilled brine or glycol refrigeration units. Thus, to meet the aforementioned temperature and pressure requirements for absorption with n-butane, the reactor effluent must first be compressed to a suitable pressure, i.e., greater than about 4.2 bara, prior to its introduction into the absorption zone. The higher pressures and resulting polytropic temperature rise within the compression zone in the presence of high concentrations of epoxybutene can cause formation of polymeric materials that deposit on the walls of the compressor and associated piping. The build-up of such polymeric material reduces the operating efficiency of the compressor and can lead to permanent equipment damage and frequent process shutdowns for maintenance, with subsequent loss of production and revenues. Moreover, the large inventory in the absorption/distillation of highly volatile and explosive butadiene and butane is dangerous and leads to higher than average safety-related costs.
U.S. Pat. No. 6,018,061 addresses the problems inherent with the compression of high concentrations of EpB, as exemplified in U.S. Pat. Nos. 5,117,012 and 5,312,931, by providing a compression or absorption refrigeration cycle for cooling the epoxybutene absorption zone prior to compression with the reaction diluent, e.g., a C3 to C5 hydrocarbon, preferably butane/butadiene, as the refrigerant. In this fashion, the epoxybutene absorption zone can be operated at pressures less than about 4 bara and a temperature of less than about 40xc2x0 C. without the need for pre-compression or external refrigeration. However, this process also has disadvantages. With pressures in the absorption zone higher than the 4 bara specified in the ""061 patent, the auto-cooling effect provided by the refrigeration cycle is greatly diminished. The temperature of the absorber becomes hotter and the absorptive power of the solvent, i.e., butane/butaidiene is greatly reduced. Thus, for example, at a pressure of 5.5 bara (80 pounds per square inchxe2x80x94psia), the auto-refrigeration effect provides only a temperature of about 60xc2x0 C. Moreover, at pressures above 4 bara, the potential for unwanted condensation of n-butane/butadiene in equipment in the recycle loop increases dramatically. Excessive condensation can cause the recycle gas composition to become flammable, an unsafe and unacceptable operating condition. Finally, as with the ""012 and ""931 patents the inventory of highly volatile and explosive butadiene and butane is large.
U.S. Pat. No. 5,618,954 discloses the recovery of epoxybutene from a butacliene epoxidation reactor effluent gas by countercurrent contact in an absorption zone using a solvent comprising water as a primary component. Epoxybutene is recovered from the water by stripping with an inert gas, similar to the conventional EO recovery process described above. As explained above, water by itself is a poor absorbent for epoxybutene and its use results in uneconomical process due to the required high water to epoxybutene ratio. Moreover, the process as described in the ""954 patent is incomplete and cannot provide purified epoxybutene. No mention is made of the binary epoxybutene-water minimum-boiling azeotrope nor of methods tc obtain purified epoxybutene from this azeotrope with water.
In view of the recovery processes described above, it is apparent that there is a need for an improved process for the efficient and economical recovery and purification of epoxybutene from the product gas of a vapor phase epoxidation reactor.
It has been discovered that epoxybutene can be recovered from a substantially vaporous epoxidation effluent comprising epoxybutene, oxygen, butadiene, and inert reaction diluent, e.g., methane, ethane, nitrogen, and the like, by intimately contacting the vaporous effluent with an effective amount of a high-boiling liquid absorbent or solvent in an absorption zone, such as an absorber, to absorb essentially all of the epoxybutene present in the vaporous reactor effluent. The present invention therefore provides a process for the recovery of epoxybutene from a substantially-gaseous effluent from an epoxidation zone wherein butadiene is contacted with an oxygen-containing gas in the presence of a catalyst and an inert diluent, to produce an epoxidation effluent comprising epoxybutene, butadiene, oxygen, an inert diluent and water which comprises feeding the effluent to an absorption vessel wherein the effluent is intimately contacted with a high-boiling, liquid absorbent to obtain:
(1) a gaseous effluent comprising butadiene, oxygen and an inert diluent from the upper section of the absorption vessel; and
(2) a liquid effluent comprising epoxybutene, the absorbent and water from the lower section of the absorption vessel; wherein the absorbent has a boiling point at ambient pressure of at least 100xc2x0 C.; epoxybutene is 3,4-epoxy-1-butene; and butadiene is 1,3-butadiene.
A second embodiment of the present invention provides for the recovery and purification of epoxybutene from the above-described substantially-gaseous effluent from an epoxidation zone by the steps of:
I. feeding the effluent to an absorption vessel wherein the effluent is intimately contacted with a high-boiling, liquid absorbent to obtain (1) a gaseous effluent comprising butadiene, oxygen and an inert diluent from the upper section of the absorption vessel and (2) a liquid effluent comprising epoxybutene, the absorbent and water from the lower section of the absorption vessel;
II. feeding the liquid effluent (2) of step I, to the middle section of a first distillation column to obtain (1) a distillate effluent comprising epoxybutene and water from the upper section of the distillation vessel and (2) a liquid effluent comprising the absorbent from the lower section of the distillation vessel;
III. allowing distillate (1) from step II to form 2 phases comprising an epoxybutene-rich phase and a water-rich phase; and
IV. feeding the epoxybutene-rich phase from step III to the upper section of an epoxybutene purification distillation column to obtain (1) a distillate effluent comprising epoxybutene and water from the upper section of the distillation vessel; and (2) an effluent comprising (a) liquid epoxybutene from the lower section of the distillation column or (b) liquid or gaseous epoxybutene from the side of the distillation column;
wherein the absorbent has a boiling point at ambient pressure of at least 100xc2x0 C.; epoxybutene is 3,4-epoxy-1-butene; and butadiene is 1,3-butadiene. Additional embodiments of the invention include the refining of the water-rich phase obtained from step II and the removal of absorbent present in effluent comprising butadiene, oxygen and an inert diluent from the upper section of the absorption vessel.